The invention relates generally to store price display systems, and relates more particularly to low-power wireless two-way communication arrangements in such systems.
An electronic shelf label (ESL) system comprises many individual, addressable ESLs in a store, typically 15,000 or more. These ESLs are situated in areas according to the organization of the retail store. Specifically, the ESLs are along a shelf edge (in some systems mounted on a rail), and several shelves are associated vertically in a vertical bay. Several vertical bays may be logically associated as a section or category, and several sections may be positioned in a half-aisle. Rail-based systems can deliver power to the ESLs, allowing extensive communications to the ESLs limited only by the communication bandwidth. However, rail-based systems are best suited to installation on shelving units, and present installation difficulties and additional costs when installed in other areas of the retail environment such as peghook displays, produce areas, free-standing display tables, deli and meat display cases, wire bins (such as for bakery products), and other general merchandise areas. For these areas, an ESL system using self powered ESLs (battery, solar, or other technologies) is highly desired because it offers installation advantages in that the ESLs may be directly attached to a variety of retail display fixtures without the impediment of wiring and the associated installation and maintenance costs.
Prior art (Pat. No. 5,241,467, Failing et. al., and Pat. No. 5,245,534, Waterhouse et. al., each assigned to the same assignee as the assignee of the present application, and each of which is incorporated herein by reference) describes means and methods to collect, maintain, and use location information on each ESL and the product it represents. This information is then used to cause all ESLs in an area or sub-area to change their displays in response to a user request initiated by a hand held unit, a special purpose module, an initiator, or a display function switch. In the current art, multiple messages must be prepared and sent, one each to each ESL in the desired area, to effect the desired display change. In a dense area, such as Health And Beauty Aids (HABA), the time necessary to address all the ESLs in a section or several adjacent vertical bays may take several tens of seconds, too long to allow for efficient in-aisle inter-activity with ESL-displayed store maintenance information, such as Computer-Aided Ordering (CAO), shelf or space management, inventory management, or promotional or merchandising information. In addition, for a power limited system, such as an RF or IR system powered by solar cells or batteries, it is desirable to minimize the number of transmissions from each ESL and, more importantly, to minimize the receiver-on time (during which the label waits for a possible message), in order to conserve power and extend battery life.
Co-pending patent application Ser. No. 08/201,470 (assigned to the same assignee as the present application and incorporated herein by reference) for Automatic Merchandising Audit Systems is directed to additional applications for shelf edge labels that also require increased communications to the labels. These applications require reasonably rapid response by the system to the operational actions being taken by the store employee in the aisle. In order to avoid long delays, and the resulting increased labor costs waiting for the label response, the receivers must all be turned on, or at least the label receivers in the geographic section in which the activity is occurring must be turned on, again consuming the energy from the battery. Co-pending application Ser. No. 08/247,334, Sub-Global Area Addressing (assigned to the same assignee as the assignee of the present application, and incorporated herein by reference), attempts to address this issue by providing means by which groups of labels may be quickly activated while minimizing the total number of labels required to have receivers on. This solution provides some benefit in trading off response time of a limited geographic area with battery life, it requires some additional software overhead to implement.
The primary limitation of a typical fully functional wireless electronic shelf label is the receiver on time. Transmissions in response to queries are typically short, perhaps tens of milliseconds, so that the total energy used in a transmission is relatively small. However, the receiver must be on for relatively long periods on order to be ready for an unexpected message, such as a price change or another display change to support in-aisle activities such as merchandising, computer aided ordering, or space management. Co-pending patent application Ser. No. 08/258,409, Low-Powered, RF-Linked Price Display System (assigned to the same assignee as the assignee of the present application, and incorporated herein by reference), addresses the power limitation by combining passive RF transceiver technology with a battery powered (or alternatively solar powered) electronic shelf label with a liquid crystal display (LCD). The technology is well known to operate such displays for the required 5 to 7, perhaps longer, years using an economical lithium coin cell. By implementing a passive RF transceiver with the ability to receive a message when energized, and alert the LCD controller through an interrupt line when receiving a message, the message can be captured by the electronic shelf label without expending any more than a minimum amount of battery energy when transferring the received data to the label. In an alternate configuration, the passive RF transceiver need not store any of the data except its own unique ID (or sub-global IDs if used), but needs only to wake up the electronic shelf label when a message is to be received.
The use of passive RF transceivers or RF diode detectors biased at acceptably low power levels requires the system transceivers communicating with the ESLs to operate at reasonably high power levels. For operation within the United States in the unlicensed ISM (instrument, scientific, and medical) bands, the radiated power is limited to less than 1 milliWatt (FCC 15.249) for unrestricted modulation implementations. Within the modulation restrictions in these bands, unlicensed radiation power levels are limited to 1 Watt (FCC 15.247). For a passive ESL transceiver, utilizing the well-known art of reflective antenna techniques for low power communications to the system transceiver, the reliability of the link is greatly dependent on the radiated power level of the system transceiver, since the energy "transmitted" by the ESL is the reflected energy received at the ESL antenna from the system transceiver. This is due to the level of received signal from the ESL at the system transceiver falling off at least as quickly as the fourth power of the range between the system transceiver and the ESL.
The radiated power level of the system transmitters can be increased, since these units are likely communicating directly with the host (by wire) and are therefore easily powered from a source connected to the AC power mains and not subjected to battery limitations. However, within the United States radiation power levels above 1 milliWatt are subject to modulation and operation restrictions, and radiation power levels above 1 Watt are not authorized. A solution is to obtain site licenses from the FCC for operation at higher power levels using the desired modulation techniques. (Not all modulation techniques are equally applicable due to the low cost and low power requirements of the ESL). Operational licenses for radiated power levels up to 8 Watts have been authorized (on an experimental basis) for this purpose. However, site licensing is not a very practical solution, since the licenses must be renewed on a regular basis (typically 5 years) on a site-by-site basis. This could present an annoying paperwork burden on a supermarket retail chain of several hundred to well over 1000 stores. Additionally, increasing health concerns by the medical authorities, government (OSHA), and consumers about the safety of microwave radiation make the decision to increase power levels arbitrarily an undesirable one. This is made worse by the fact that the desire would be to select a physical location for the system transceivers that is as low as practical in order to reduce the effects of the path loss (fourth order of range), thus placing the transmitters even closer to the consumers and store employees. An alternate solution might be to operate at lower, therefore safer, power levels, but this would require that additional system transceivers be installed in the ceiling, thus increasing the acquisition, installation, and maintenance costs of the system.
Modulation techniques used by the current technology of passive communication systems using the reflective impedance of antennas have additional limitations in the system transceiver. Typically, these systems use a double-sideband amplitude modulated (DSB AM) signal for the downlink from system transceiver (transmitter) to the ESL transceiver (receiver or detector). For uplink communications, the system transceiver continues to transmit a continuous wave unmodulated carrier signal and in the ESL, an RF device modulates the impedance of the ESL antenna with a local oscillator. Oscillator frequencies of 10 to 20 kHz are typical. The data are transmitted by applying double-sideband AM modulation to the local oscillator, thus creating modulated sidebands on either side of the carrier signal separated by the local oscillator frequency. Typical local oscillator frequencies would be about 16 kHz (easily obtained from low-cost 32.768 kHz watch crystals).
The system is further complicated by the fact that the received reflective modulation signal will typically have a signal level of less than -30 dBm, while the transmitted unmodulated carrier, which has to be operating simultaneously in order to produce the reflective modulation, will typically be operating at a power level of about +30 dBm. Since the path loss for this transmission falls off at the fourth power of range, combined with the desire to minimize the total system costs (acquisition and installation), the system transmitter and the system receiving antenna must be located in close proximity. This means that the receiver must reject a carrier signal 60 dB higher than the maximum expected receive signal, and the carrier signal distance in frequency from the center of the receiver bandwidth is of the order of the bandwidth itself. This requires extremely steep skirts on the receiver bandpass filter, or an accurate notch filter, and precludes relaxation of the carrier frequency tolerance for cost reduction purposes. Accommodation is limited, because moving the receive antenna closer to the transmitter increases the interference from the carrier, and increasing the separation reduces the received reflective signal.
What is desired is a transmission scheme that radiates at lower power levels to be safe for consumers and store employees, does not require site licensing, has reasonable coverage to minimize the number of system transceivers needed to cover an area of interest (the store), and still provides reliable communications at low cost and low power levels with a battery powered ESL (electronic shelf label).